Yeah it may or may not, depending on your game/server etc and how the GUI is designed. And also what fonts you have loaded prior to it being called.
This wasn't just posted for an FYI. Its in response to the issues it has caused others, so be fairly warned

If SubstituteFont() fails, its a ctd. (this tested on a clean engine world)
What prompted this post was, a major issue on other worlds with crashes upon calling it.
To be safe (unless you are already successfully doing so) don't call within the first chance as GUI's/fonts are not guaranteed to be loaded yet.
Also a further note, the font enum value you're addressing to substitute should be one of the default fonts such as "DEFAULTTEXT"/"GAMETEXT" etc., otherwise it will crash the blade/client. At one time you could add a new value to the "font" enum and substitute that value with your custom font. this is no longer the case.
The reason for this notice and the caution against using it in a function such as FirstChance() is due to the nature of the FirstChance() function being loaded on client load up. If it fails, you'll have to deal with reverting the proper script in the repo to get the client working again, which can be a mess.
Safest Option: Don't do eet

If you have it working, I would still take some precautions and at the very least ensure SubstituteFont() isnt called prior to loading to Character Select so that you always have the ability to login and make changes as needed.
This isn't something caused by the latest update, this is something that simply hasn't been used by many people and hasn't seen the testing that much of the other systems have over the last year, and so just now got really noticed.
Thanks